Tubular heating elements and magnesia insulation therefor and method of production

ABSTRACT

The compaction density and thermal conductivity of granular, fused magnesia used as thermally conducting electrical insulation in tubular, electrical resistance elements is substantially improved by controlled surface hydroxylation of the magnesia powder before compacting it in a heating element in the usual manner.

United States Patent Willem Vedder Latham, NY.

Mar. 2 1, 1969 Nov. 23, 1971 General Electric Company Inventor Appl. No. Filed Patented Assignee TUBULAR HEATING ELEMENTS AND MAGNESIA INSULATION THEREFOR AND METHOD OF PRODUCTION .8 Claims, 3 Drawing Figs.

US. Cl 219/544, 174/118, 219/548, 219/553. 252/632, 252/635, 338/238 Int. Cl 1101c 1/02, HOSb 3/10, H05b 3/44 Field of Search 252/632,

References Cited UNITED STATES PATENTS 3,477,058 11/1969 Vedder et al. Primary Examiner-John T. Goolkasian Assistant Examiner-Robert A. Dawson Attorneys-Paul A. Frank, Richard R. Brainard, Charles T.

Watts, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT: The compaction density and thermal conductivity of granular, fused magnesia used as thermally conducting electrical insulation in tubular, electrical resistance elements is substantially improved by controlled surface hydroxylation of the magnesia powder before compacting it in a heating element in the usual manner PAIENTEnuuv 23 l97| Compaction Densit in Tnearericai Bound Water in ppm Fig.

Fig. 2

O 86% O o O O O O as 0 o 0 83 l I r 1 1 v r Bound H 0 in ppm 2000- Fig. 3.

0 I I I I l Time in Minutes inventor Wi/le Vedder Mix M His Attorneyhigh-frequency current heating devices.

TUBULAR HEATING ELEMENTS AND MAGNESIA INSULATION THEREFOR AND METHOD OF PRODUCTION The present invention relates generally to tubular, electrical-resistance, heating elements and is more particularly con- "cerned with a novel method of making such elements includsistance wire. structurally, it usually includes: (1) a coiled resistance wire composed of alloys such as those made up of 20 percent chromium and 80 percent nickel; (2) compacted magnesia powder containing minor amounts of-impurities surrounding the resistance coil as an insulator; and (3) an outer protective metaljacket.

Over the long period in which such elements have been in general use, they have been-developed and improved to a state of good performance and service life, meeting high safety standards and competing with consistent success with gas and At the same time, however, it has long been recognized that a substantial increase in the thermal conductivity of the magnesia insulation employed in these elements would be desirable. This objective, however, would have to be realized without incurring any substantial offsetting disadvantage of increased production or operation cost or reduced efficiency.

In accordance with this invention, based upon my discovery subsequently to be described, this objective has been achieved. Moreover, no substantial modification 'of theprincipal operations involved in the commercial production of tubular heating elements is required in order to obtain the benefits of this invention.

In its method, article and composition aspects, this invention is predicated upon my discovery that by hydroxylating the individual particles of a magnesia powder to a limited extent under controlled conditions, the density attained during compaction of the powder can be substantially increased with resulting improvement in the heat conduction in tubular-electrical-resistance heaters incorporatingthese powders. In fact, compaction densities approximating those of the novel com positions disclosed and claimed in copending U.S. Pat. applications Ser. No. 702,474, now U.S.-Pat. No. 3,477,058, filed Feb. 1, 1968 in the names of Willem Vedder and John Schultz, Jr.; and Ser. No. 702,166, filed Feb. 1, 1968 in the name of Louis Balint, both of which are assigned to the assignee hereof, can thus be obtained without the use of any of the additives disclosed in those copending cases.

In some way not totally understood the surface modification of magnesiapowder particles in accordance with the present invention imparts to them lubricity which minimizes particle fracture during the compacting operation and increases both the compaction density and the thermal conductivity of the magnesia insulation in the tubular heater.

This invention in its composition aspect accordingly in general comprises a mass of granular fused magnesia powder particles containing bound water and carbon dioxide in their surface portions in amounts imparting superior compaction characteristics to the mass. Specifically, the mass contains 180 to 2,000 parts per million of bound water and up to 500 parts per million of bound carbon dioxide, and will preferably consist of freshly crushed and hydroxylated fused magnesia of 40 +65 mesh size (U.S. Standard screen sizes). The powder mass may, however, include a wide variety of particle sizes from 40 mesh to below 325 mesh.

In its article aspect, this invention, generally described, comprises a tubular heating element including a metal sheath, a coaxial coiled resistor in the sheath and a compacted,

polycrystalline mass of a magnesia composition of this invention filling the space between the resistor and the sheath. Those skilled in the art will understand that this description of the article applies to the article at the intermediate stage of its production when the composition of this invention has been introduced into the sheath but prior to the time when the article has been thermally cycled.

Finally, in its method aspect, this invention, described broadly, involves the use of the novel composition described above in the production of a tubular heating element including particularly the step of filling the metal sheath with that novel material. Thus, this method centers in a use concept which in itself has novelty independently of the uniqueness of the composition per se.

Referring to the drawings accompanying and forming a part of this specification:

FIG. l is an enlarged, side-elevational view of the heating element of this invention, portions being broken away for purposes of illustration;

FIG. 2 is a chart on which compaction density (in percent theoretical) is plotted against bound water content (in parts per million) for a number of magnesia powder samples; and,

FIG. 3 is a chart on which bound water (in parts per million) is plotted against treating (hydroxylation) time in minutes on a square root scale.

The heating element of FIG. 1 resembles the heretofore conventional tubular heaters in that it is made up of three principal parts. Thus, a coiled resistance wire 1 is disposed within an outer protective metal jacket 2 and is embedded in and spaced from the jacket by compacted magnesia powder 3 which serves both as a thermal conductor and electrical insulator. In contrast to the prior devices, however, the heating element of FIG. 1 incorporates magnesia powder of this invention which has superior thermal conductivity because of its compaction density which in turn is attributable to its critical bound water content.

The FIG. 1 element is suitably fabricated in accordance with the usual practice in the art, theparts being assembled and the element being conditioned at elevated temperature. Thus, essentially the only significant departure from prior practice in terms of the fabrication operation consists in the use of the new magnesia compositions of this invention, these being substituted for magnesia used in accordance with the prior art-practices in order to obtain the special new results and advantages stated above.

In the practice of this invention, fused magnesia is crushed and the resulting powder is hydroxylated so that hydroxyl groups are introduced into the surface layer of each powder particle. The hydroxylation step is carried out in room air at saturated humidity and containing carbon dioxide in normal proportion. Preferably, the magnesia is of particle size less than 40 meshandhydroxylation is done at C. or higher and up to 175 C. The time required for a given degree of hydroxylation will depend upon the temperature to the extent that, for instance, the hydroxylation result obtained in 5 hours at C. can be obtained in 5 minutes at C. In any case, however, the amount of bound water in the hydroxylated powder product must be critically controlled to between and 2,000 parts per million and the bound carbon dioxide content must' not exceed-500 parts per million. The consequence of having too much bound water or carbon dioxide is that compaction density is diminished to the level of that of the unhydroxylated magnesia powder. Too much bound water means that the magnesia surface is covered with a layer of magnesium hydroxide, i.e. the powder particle surfaces are hydrated as distinguished from being hydroxylated." Spalling of the hydroxide apparently largely accounts for the poor compactioncharacteristics, but the presence of magnesium carbonate in the particle surfaces may be a contributing factor. Thus, the addition of magnesium hydroxide to untreated (i.e. neither hydroxylated nor hydrated) magnesia powder has been found to have a negligible effect on the density of the powder mass after compaction.

The following illustrative, but not limiting, description of experimental work is offered in the interest of insuring a full and clear understanding of this invention by those skilled in the art and enabling their practice of it without the necessity for experiment to obtain the new results and advantages stated above.

Single crystals of Norton Company periclase about threefourth inch in each dimension were crushed in a tool steel percussion mortar with a minimum number of blows, screened mechanically, and the portion passing 40 mesh and retained on 60 mesh was selected for experiments. Optical examination indicated a clean separation in this size range with no finer particles than 60 mesh in the mass. Portions of the crushed material were placed in lO-gram batches in coarse-mesh silk bags and then heated in two different groups (one at 80 C. and the other at 95 C.) in an atmosphere of room air at saturated humidity at the treating temperature. At intervals of time, individual bags were removed from the heating chambers and half of the powder in each bag was used for analysis of bound water and carbon dioxide, and the other half was used for pressing l-gram pills in a -inch carbide die at a pressure of 120,000 p.s.i. These pressings and analyses were done promptly to minimize any reaction with the atmosphere. Control specimens of 40 +60 mesh particles which had not been exposed to water were pressed at the same time and in the same way. The selected specimens were weighed and measured for determination of compaction densities. The results obtained in these tests are shown on the chart of H6. 2 where each point represents a single specimen.

The specimens were also examined optically under magnification with the result that it was found that the microstructures of the unhydroxylated compacts differed markedly from that of the compacts of hydroxylated magnesia. This difference is the fracturing of the particles, the unhydroxylated material revealing extensive particle fracture and fragmentation while the hydroxylated magnesia is only comparatively slightly fragmented in this way.

Tests of these specimens for absorbed or bound water yielded the data points illustrated by curve A (80 C. series) and curve B (95 C. series).

Tubular heater units made through the use of the hydroxylated magnesia product of this invention prepared as set forth just above have better thermal conductivity than those made through the use of the unhydroxylated magnesia of these experiments. Thus, there is a direct relationship between com paction density of magnesia and thermal conductivity of the magnesia in a tubular heating element just as there is between bound water content and compaction density of the magnesia. But there does not appear to be any significant dependence of compaction density upon the manner in which or the particular conditions under which the magnesia is hydroxylated.

In another embodiment of this invention, the tubular heating unit is made through the use of the hydroxylated magnesia product of this invention prepared as set forth above, the magnesia mass containing from 1.0 percent to 5.0 percent of pyrophyllite which is substantially uniformly distributed through the magnesia mass.

Wherever in this specification and in the appended claims reference is made to percentages or proportions, reference is had to the weight basis rather than the volume basis unless otherwise specifically stated.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the method of making a tubular heating element which includes the steps of positioning a resistor within and spaced from a metal sheath and filling the metal sheath with a polycrystalline mass of magnesia, the combination of the preliminary step of hydroxylating the magnesia mass substantially uniformly throughout by subjecting the said mass at elevated temperature to contact with a moisture-containing atmosphere until said mass contains from 180 to 2,000 parts per million of bound water and not more than 500 parts per million of bound carbon dioxide.

2. The method of claim 1 in which the mass is at a temperature of about C. during the hydroxylation step.

3. The method of claim 1 in which the moisture-containing atmosphere is air.

4. The method of claim 1 in which mass contains about 800 parts per million of bound water upon completion of the hydroxylating step.

5. The method of claim 1 in which the magnesia mass is freshly crushed fused magnesia of 40 +60 mesh particle size.

6. In a heater element including an electrically resistive heater coil and a metal sheath surrounding and enclosing and spaced from the coil, the combination of a mass consisting of fused polycrystalline magnesia filling the space between the coil and the sheath and electrically insulating the sheath from the coil, said magnesia mass containing from to 2,000 parts per million of bound water and not more than 500 parts per million of bound carbon dioxide.

7. The heater element of claim 6 in which the magnesia mass contains from L0 percent to 5.0 percent of pyrophyllite substantially uniformly distributed through the magnesia mass.

8. A composition of matter having special utility as a thermally conducting and electrically insulating filler for sheathed electric-resistance heaters consisting essentially of a mass of granular fused magnesia powder particles containing from I80 to 2,000 parts per million of bound water and not more than 500 parts per million of bound carbon dioxide, the said water and carbon dioxide being distributed substantially uniformly throughout the mass and being contained essentially only in the outer portions of magnesia particles. 

2. The method of claim 1 in which the mass is at a temperature of about 100* C. during the hydroxylation step.
 3. The method of claim 1 in which the moisture-containing atmosphere is air.
 4. The method of claim 1 in which mass contains about 800 parts per million of bound water upon completion of the hydroxylating step.
 5. The method of claim 1 in which the magnesia mass is freshly crushed fused magnesia of -40 +60 mesh particle size.
 6. In a heater element including an electrically resistive heater coil and a metal sheath surrounding and enclosing and spaced from the coil, the combination of a mass consisting of fused polycrystalline magnesia filling the space between the coil and the sheath and electrically insulating the sheath from the coil, said magnesia mass containing from 180 to 2,000 parts per million of bound water and not more than 500 parts per million of bound carbon dioxide.
 7. The heater element of claim 6 in which the magnesia mass contains from 1.0 percent to 5.0 percent of pyrophyllite substantially uniformly distributed through the magnesia mass.
 8. A composition of matter having special utility as a thermally conducting and electrically insulating filler for sheathed electric-resistance heaters consisting essentially of a mass of granular fused magnesia powder particles containing from 180 to 2,000 parts per million of bound water and not more than 500 parts per million of bound carbon dioxide, the said water and carbon dioxide being distributed substantially uniformly throughout the mass and being contained essentially only in the outer portions of magnesia particles. 